Method and apparatus for the treatment of hard biological material, such as hard dental material, using lasers

ABSTRACT

A method and apparatus for the ablation of hard biological material, such hard dental material, using a rapidly pulsed laser employ a crystal or absorber film disposed in the propagation path of the laser radiation, the crystal or absorber film being disposed at the laser output and serving as an interface with an optical conductor which conveys the laser radiation to the treatment site. The crystal or absorber film smooths the time/intensity characteristics of the pulsed laser radiation to the extent that transmission to the treatment site using optical wave guides is possible. Additional protective features are provided to prevent the ablated biological material to destroy the exposed optical end surfaces of the treatment applicator. Additional design features improve the ergonomic efficiency of the handpiece and its ability to be sterilized.

This is a continuation-in-part, of application Ser. No. 07/934,771,filed Aug. 24, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a laser-based method and apparatusfor the treatment of hard biological material, such as the ablation andremoval of hard dental material.

2. Description of the Prior Art

Processes and devices employing a pulsed laser for removing hardbiological material, such as hard dental material are described, forexample, in German OS 4 030 734 and OS 3 911 871. In OS 4 030 734, aparticular technique, and various types of equipment for implementingthe technique, are described for treating carious teeth and forconducting root canal repairs. The devices described therein, ingeneral, include a pulsed laser, a fiber optic transmission system, anda fiber optic laser-transmitting handpiece with interchangeable therapyheads. The source of the radiation is a pulsed alexandrite solid-statelaser, operating in the wavelength range between 720 and 860 nm. By theaddition of an appropriate optical module, it is possible to double thefrequency, and therefore attain an operating range between 360 and 430nm. In one embodiment of this known system, the laser beam is emittedfrom the removable head as a "free" beam, that is, the focus liesoutside the emission plane. In another embodiment, the emission point isat the end of a light waveguide which is located in the therapy head.The laser beam is emitted as a dispersing beam in this embodiment. Thisembodiment is designed specifically for the preparation of root canals.The coupling into and out of light guides is generally accomplishedusing spherical lenses. At the point of termination, the face of boththe handpiece and the therapy head contain a window made of compressedquartz glass having anti-reflective properties. This prevents dust anddirt from entering the individual components of the equipment.

In OS 39 11 871, a process is described for the removal of dentalmaterial using a pulsed infrared laser. In this known method, the dentalmaterial is covered with a thin film consisting of a fluid which absorbslaser radiation, either before or during the irradiation process. Bythis technique, the danger of damage to the surrounding healthy tissueis reduced, while not interfering with the efficiency of the removal ofdental material. The fluid is applied intermittently, i.e., during thepauses between the respective laser pulses.

It is generally known that, using a pulsed laser system, the individualpulses of the laser radiation may exceed the threshold of criticalenergy concentration (which varies by material), so that biologicalmaterial can be removed without creating a significantly increasedtemperature in the areas peripheral to the treatment location. Toachieve these results, however, extremely short light pulses (on theorder of nanoseconds) must be used, and the thickness of the biologicalmaterial removed by this method is between 10 and 50 microns. To reachworthwhile rates of material removal, given such a tiny thickness perindividual light pulse, it is necessary to increase the repetition rateof the laser pulses. Because hard biological material has a limited heattransfer capacity, however, increasing the repetition rate of the pulsesrapidly leads to an accumulation of heat around the zone of removal, andhence leads to thermal damage of the areas peripheral to the treatmentarea. To reduce the level of thermal damage resulting from the use oflaser systems wherein the laser radiation is not significantly absorbedby naturally-present water or air, various types of cooling equipmentare known which generate a continuous jet of water or a continuous flowof air to the treatment site.

It is known that transmitting high intensity laser radiation throughoptical fibers can cause a photohydraulic phenomenon at the treatmentsite, which causes the biological material which has been ablated by theaction of the laser pulses to impinge on the exit face of the lightguide, thereby resulting in the destruction of the light guide fibers.This, in turn, requires that the treatment be terminated. In an effortto avoid this problem, the transmission of such high intensity lightlevels is usually accomplished by using movable arms which incorporatemirrors. Such a transmission method is disclosed, for example, in theGerman OS 39 11 853. This document shows an opto-mechanical endpiece forone such mirror arm for an Er:YAG laser. This arrangement permits thetransmission of pulsed laser radiation,, however, such mirror systemshave the disadvantage that any error in the assembly, or in thedirection of the output of the laser radiation, is increased by a factorof two with each mirror present in the apparatus. Therefore, anydeviations in the coaxiality of the system resulting from misalignmentsat the optical interface locations, or dynamic variations in the outputdirection of the laser radiation, or its angle of dispersion, aremultiplied by two for each point at which the light changes direction.Such mirror systems are, therefore, extremely difficult to adjust.Moreover, the cost of manufacturing the mechanical parts with thenecessary precision is, as a result, very high. Additionally, suchmirror systems are freely movable only within limits, becauseconsiderable force is required to articulate each point of directionalchange, and any two points cannot always be connected with each otheralong the shortest path. Although it is theoretically possible totransmit optical radiation using light waveguides, the ability totransmit high intensity short laser pulses, under conditions asdescribed above, is limited by the technical capabilities of thematerial currently available for the manufacture of optical fibers.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus for treating hard biological substances, such as bones orteeth, (as an alternative to the conventional dental procedures ofdrilling or grinding) without generating thermal damage in areasperipheral to the treatment area, those peripheral areas being at agreater depth than the optical depth of penetration of the laserradiation.

It is a further object of the present invention to provide such a methodand apparatus wherein the rate of removal of the biological material is,in terms of volume removable per unit of time, comparable to ratesachievable using conventional procedures.

It is another object of the present invention to provide such a methodand apparatus which can be used not only in a laboratory environment,but also for human treatment, in which case the opto-mechanicalendpieces of the apparatus must be capable of being easily sterilized.

Another object of the present invention is to provide a method andapparatus wherein the opto-mechanical endpiece of the equipment isdesigned to be hand-held and is freely movable in all directions.

The invention is based on the perception that, surprisingly, thethreshold for volume destruction of optical fibers is significantlyhigher than the threshold at which surface destruction takes place underbombardment with high-intensity laser light. The surface destructionthreshold increases approximately in proportion to the square root ofthe pulse length. This means that, in the borderline case of a laserwhich is continuously irradiating, the surface and volume destructionthresholds move closer together. In order to meet the criterion ofhaving the smallest possible amount of damage in the areas peripheral tothe treatment area, only lasers having emission wavelengths of eitherless than 400 nanometers, or greater than 1.1 microns, can be used.Taking the further criterion into account, that the removal rates mustbe comparable to those achievable using conventional systems, theapplicable lasers are only those with pulse lengths which lie betweenapproximately 0.5 and 500 microseconds. Considering the absorption curveof hard dental materials or bone, working wavelengths in the respectiveranges of 200 through 400 nm, 1.3 through 3 microns, and 9.0 through 11microns can be used. In a preferred embodiment, a solid-state laser isused having an emission wavelength of 2.78 microns. Ideally, this wouldbe a YSGG crystal laser (Yttrium Scandium Gadolinium Garnet) with Cr-Erdoping. Alternatively, good results can be obtained using an Er:YAGlaser which emits radiation at a wavelength of 2.94 microns. Laserswhich emit radiation between these two wavelengths (i.e., between 2.78microns and 2.94 microns) are preferentially acceptable. The laser isused with media which achieves smoothing of the time intensitycharacteristics of the laser pulses, so that transmission of thosepulses using optical waveguides is viable.

The medium which achieves smoothing of the intensity characteristics ofthe laser pulses which vary over time may alternatively be an absorberfilm formed by solid or fluid material, the material being so-called"self-healing" material.

It is surprising that the ablation which takes place at the treatmentsite causes the absorption maximum of the target material to dynamicallyshift to shorter wavelengths, and that this occurs even during the timewhen a pulse laser is in actual operation. In combination with thedesign measures described below, this surprising revelation can be usedto achieve a system for treating hard biological material wherein thelight can be transmitted to the treatment site by an optical waveguide,without damaging the optical waveguide to an extent requiring thetreatment to be stopped. In the specific example of water-containinghard dental material (hydroxylapatite), its absorption maximum shiftsfrom around 3 microns to approximately 2.8 microns. Thus, when thepreferred wavelength is used, the process "optimizes" itself during thecourse of each individual pulse.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the emission performance over time of a pulsedsolid-state laser.

FIG. 2 is a graph of the emission characteristics achieved in accordancewith the principles of the present invention, which are ideal fortransmitting high energy pulses.

FIG. 3 is a schematic representation of a laser resonator with anintensity smoothing element disposed inside the resonator, in accordancewith the principles of the present invention.

FIG. 4 is a schematic representation of a laser resonator with anintensity smoothing element disposed outside of the resonator, inaccordance with the principles of the present invention.

FIG. 5 is a side view, partly in section, of an apparatus for treatinghard biological material constructed in accordance with the principlesof the present invention, in the form of a dental instrument.

FIG. 6 is a side sectional view of a portion of the light waveguide tubein the apparatus of FIG. 5.

FIG. 7 is a side sectional view of the dental handpiece in a furtherembodiment for transmitting the laser light to the treatment end of thehandpiece.

FIG. 8 is a side sectional view of the dental handpiece in anotherembodiment for transmitting the laser light to the treatment end of thehandpiece, using a flat mirror.

FIG. 9 is an enlarged side sectional view of the treatment end of afurther embodiment of a dental handpiece, employing a curved mirror.

FIG. 10 is an enlarged side sectional view of the treatment end of adental handpiece in a further embodiment also using a curved mirror,with a different type of treatment tip.

FIG. 11 is an enlarged side sectional view of the treatment end of adental handpiece, in a further embodiment employing a curved mirror.

FIG. 12 is an enlarged side sectional view of the treatment end of adental handpiece in a further embodiment employing a prism with atreatment tip of the type shown in FIG. 10.

FIG. 13 graphically shows the adaptation of air-to-fluid ratio used inaccordance with the principles of the present invention.

FIG. 14 is a schematic showing of the arrangement of valves foroperating the apparatus in accordance with the principles of the presentinvention.

FIG. 15 is a timing diagram for the control of the fluid/air mixture(aerosol) in the operation of the apparatus according to the principlesof the present invention.

FIG. 16 is an enlarged side sectional view of the treatment end of thehandpiece shown in FIG. 5.

FIG. 17 is a schematic representation of a laser with an intensitysmoothing element in the form of an absorber film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The emission performance over time of a conventionally operated pulsedsolid-state laser is shown in FIG. 1. As a result of the spikes, whichare greatly amplified compared to the average energy level, very highconcentrations of energy will be present at the surface of the opticalfibers, which are used to conduct the laser light, at the point ofinterface with the laser radiation. Even when the levels of energyconcentration are too low to cause actual volume destruction of thefibers, the fiber surface is nonetheless frequently destroyed at thetime of coupling of the radiation to the light waveguide, and the systemwill therefore malfunction.

Ideal emission characteristics for transmitting high energy pulses, asare achieved in accordance with the principles of the present invention,are shown in FIG. 2. In accordance with the invention, the technicalproblem of easily transmitting the light from a pulsed laser usingoptical fibers is solved by placing a medium in the region of highestlaser beam density which "smooths" the intensity characteristics of thebeam with respect to time. In a preferred embodiment, this medium is acrystal with non-linear optical characteristics (which will generateharmonics at several times the base frequency), which is placed in thecenter of the laser beam, either inside or outside of the laserresonator (cavity). Above certain threshold values of energyconcentration, the crystal generates the harmonics of the basewavelengths with a high degree of efficiency. The crystal may consist,for example, of lithium-iodate, lithium beta borate, lithium niobate,niobium beta borate, barium beta borate or silver gallium sulfide. Incombination with further design steps with respect to the optics, asdescribed below, or by way adding optical elements in the propagationpath of the beam, the region of highest beam density attains such alevel of concentration for the chaotic emission pulses that, above agiven energy concentration level corresponding to the average value of apulse, the intensity spikes are doubled in frequency. Such intensityspikes are then no longer amplified.

Two highly schematic embodiments of a laser with the aforementionedintensity smoothing medium, in accordance with the principles of thepresent invention, are shown in FIGS. 3 and 4. In the embodiment of FIG.3, the laser resonator has two confocal (convex curved) mirrors 1 and 2,between which a lasable substance 3 is disposed. The lasable substance 3can be a gas, fluid, or a solid such as a semiconductor. The intensitysmoothing medium 4, such as a crystal, is disposed inside the resonator.The intensity smoothing medium 4 has the characteristics as describedabove. Inside the resonator, the harmonics are not further amplified,because of the frequency-selective amplification characteristics of thelasable medium 3. In the embodiment of FIG. 3, the impulse spikes are infact clipped, although the base wavelength continues to be amplified. Anarrangement as shown in FIG. 3, with the medium 4 disposed within theresonator, causes the lowest possible losses.

It is also possible, however, to dispose the medium 4 outside of theresonator, as shown in FIG. 4. In the embodiment of FIG. 4, theresonator has a semi-reflective plane mirror 5 disposed at one endthereof. With this arrangement, the conditions for generating theharmonics can, with good results, be more easily selected and adjusted.The embodiment of FIG. 4 also includes a dichroic beam splitter 23,which reflects the intensity spikes of the frequency-doubled laserpulses, so that those spikes are not present in the output beam, whichpasses through the beam splitter 23. With such a smoothed intensityprofile, using an Er:YSGG pulsed laser, laser pulse energy levels ofmore than 500 mJ, at impulse widths of around 180 microseconds, can betransmitted using the latest available optical fibers. It will beunderstood that any other non-linear optical mechanism can be used toclip the impulse spikes, without departing from the inventive conceptdisclosed herein, in order to achieve an optimal interface of thehigh-intensity light pulses with the light transmitting medium. Anotherexample of such a mechanism is a two photon absorber.

Details of a practical embodiment of the invention are shown in FIG. 5in the form of a dental apparatus. In the apparatus of FIG. 5, laserlight is generated by a laser generator, generally referenced 6,constructed and operating either as shown in FIG. 3 or FIG. 4. Thislight is supplied to a dental handpiece 7. The light output beam at theresonator of laser generator 6 is directed to the center of a firsttwist connector 8, which transmits the light via an optical interface,which is not further described. The light then passes through a flexiblelight waveguide tube 9 to a second twist connector 10, which is in turnconnected to the handpiece 7. The handpiece 7 can thus easily be removedfrom the end of the waveguide tube 9.

From the second twist connector 10, the laser light is transmitted as a"free" beam via optical mirrors and prisms 11, 12, and 13, untileventually reaching an applicator endpiece 14, serving as the treatmenttool.

Two conduits 15 and 16, described in greater detail below, arerespectively used to carry gas (preferably air) and a liquid (preferablywater) to the treatment end of the handpiece 7. The gas conduit 15 isconnected at one end to a gas container 25, in a manner described indetail below and is connected at its other end to the liquid conduit 16via a three-way connector. The gas and liquid are mixed to create anaerosol, and the output nozzle 16a at the end of the liquid conduit 16is directed toward the treatment area such that the aerosol is sprayeddirectly onto the area being treated by the laser radiation, and notonto the surrounding area. A section, designated VI in FIG. 5, of thelight waveguide tube 9 is shown in enlarged sectional view in FIG. 6. Ascan be seen in FIG. 6, the optically transmissive material 17 iscentrically arranged and wrapped by an jacket 18 consisting ofelastomeric material. The light conducting material 17 is preferablyzirconium fluoride glass and may be arranged to form a light waveguideconsisting of either of a single fiber, or in the form of a multiplefiber arrangement (fiber bundle). This material is relatively flexible,but is still sensitive to torsion, and it is therefore important toinsulate the material 17 from torsion forces, or to minimize theireffect, for which reason the elastic jacket 18 is used. The viscoelasticproperties of the jacket 18 insure that shear or torsion forces areadequately distributed over the boundary surface. Preferably siliconrubber is used for the jacket 18. An outside jacket 19 surrounds thejacket 18. The outer jacket 19 has some degree of flexibility, however,is protected against torsion by a fabric or mesh woven into the jacket19.

As noted above, at each end of the waveguide tube 9 are respective twistconnectors 8 and 10. The twist connectors 8 and 10 are constructed sothey can freely turn around the longitudinal axis of the waveguide tube9, while still connected to the corresponding receptacle, either at thelaser side or the applicator side of the waveguide tube 9.

The practical embodiment shown in FIG. 5 employs the aforementioneddesign criteria to reduce the probability of surface destruction of theends of the light conducting material 17 and meets the requiredtransmission characteristics for Er:YSGG or Er:YAG laser radiation. Theadditional requirement of achieving an easily sterilizableoptomechanical endpiece is met as follows. The pulsed radiation from thelaser generator 6 emitted from the light conducting material 17 in thewaveguide tube 9 is passed through the rotary quick connector 10 to thehandpiece 7 which has a hollow interior. The handpiece 7 can beseparated from the waveguide tube 9 for the purpose of sterilization.The ability to be sterilized is achieved by using only a few opticalcomponents, which are not permanently attached together.

FIGS. 5 and 7 through 12 show various embodiments for passing the laserradiation through the interior of the handpiece 7 to the appropriateoptical elements disposed at the applicator endpiece 14. All of theembodiments of the handpieces 7 shown in the drawing have the advantageof a removable applicator endpiece. This applicator endpiece can eitherbe supplied in sterile packaging, or may itself be sterilized.

In the embodiment of FIG. 5, the laser light is transmitted via twoconjugated coaxial imaging lenses 11 and 12. The lenses 11 and 12 arepreferably meniscus lenses consisting of sapphire. An image of theoutput aperture of the light conducting material 17 is thus incident ona input face (surface) of a prism 13, which changes the propagationdirection of the laser light by approximately 90° and passes the laserlight to the entry aperture of the application endpiece 14. Theapplicator endpiece 14 or, briefly "applicator" is in the form of ahollow guide. The prism 13 is placed in the propagation path of thelenses 11 and 12 such that total reflection of the laser radiationoccurs on the hypothenuse surface of the prism. This allows the beam tobe displaced by 90°. As shown in greater detail in FIG. 16, the prism 13is seated on a gas container 25. Gas (preferably air) is supplied tothis container 25 through the gas conduit 15, in accordance with theinvention. The gas container 25 has a screw connector 26 which permitsdifferent applicator endpieces 14 of varying lengths and opening typesto be interchanged with one another.

A portion of the compressed gas used to form the aerosol is diverted tothe gas container 25, and is expelled therefrom through a channel 27 inthe applicator 14. The gas container 25 achieves a uniform pressuredistribution of the gas flow through the channel 27, creating a laminarflow at a ratio of at least 1:10 between the inner diameter and thelength of the flow capillaries. Due to the aforementioned photohydraulicphenomenon associated with the particles of biological material whichhave been ablated by the action of the pulsed laser, such particleswould normally tend to be directed back toward the source of the pulsedlaser radiation. In accordance with the principles of the presentinvention, however, these particles are not permitted to reach theoptical surfaces of any of the beam direction-changing elements 13, 20,(FIGS. 8, 12) or 21, (FIGS. 9, 10, 11) but are instead expelled to theside by the gas flow through the channel 27. As a result, the action ofthe gas dynamically protects the sensitive surfaces of the applicatorendpiece 14, as well as those of the other components in the handpiece7, from being damaged by the products of the ablation process. If a gashaving a higher refractive index than air is used, the gas itself canhave the ability to transmit light, including the laser radiation over ashort distance at the end of the hollow applicator endpiece 14. Thispermits the operator to work in a non-tactile mode, and also affordsaccess for the aerosol cleansing medium.

In a preferred embodiment, interchangeable applicator endpiece 14 is acapillary having a gold-plated interior, which can be removed andexchanged similar to drill bits by a dentist.

The embodiments of FIGS. 7 and 8 include a stiff metal jacket 24,surrounding an extension of the light conducting material 17 from thewaveguide tube 9. The stiff metal jacket 24 is mechanically attached atone end to the twist connector 10, and supports the light conductingmaterial 17 in the interior of the handpiece 7. The jacket 24, and thelight conducting material 17 therein, terminate close to the beamdirection-changing element in the interior of the handpiece 7. As shownin FIG. 7, this may be an optical prism 13 or, as in FIG. 8, adirection-changing mirror 20. To avoid reflection losses at the face ofthe prism 13, it is preferable to introduce immersion fluid between theend surface of the light conducting material 17 and thedirection-changing element. In the embodiment of FIG. 7, this immersionfluid is contained in a capsule 28 in which the end of the metal jacket24 is disposed. If the light conducting material 17 is in the form offibers having a suitably small numerical aperture (such as smaller than0.2), the emitted radiation can be passed directly via thedirection-changing element into the interchangeable hollow guide formingthe application tip 14.

As shown in FIG. 8, the direction-changing mirror 20 is a plane mirror.In the event that the numerical aperture of the light conductingmaterial 17 is too large to permit the laser radiation to be completelydirected into the hollow guide (taking the geometrical distances intoaccount), an angular or curved mirror 21 may be used instead of theplanar mirror 20. Such an arrangement is shown in FIG. 9. The mirror 21causes the radiation to converge in such a manner as to correspond withthe entry aperture of the hollow guide in the application tip 14. Inthis embodiment, the gas container 25 is closed by an opticallytransmissive window 29, which also facilitates the easy replacement ofthe application tip 14. Preferably the optically transmissive window 29is a thin mica pane.

In a further embodiment as shown in FIG. 10, the optics include a lens22, preferably a meniscus lens, in combination with an toroidal mirror21. In this embodiment, the laser radiation is directed into a conicalsapphire tip 30. The tip 30 is housed in a metal cone 31 to protect itfrom dirt. The metal cone 31 is held in place at the application end ofthe handpiece 7 by a screw thread, and can easily be removed andreplaced. Because of its hardness and good optical transparency,sapphire is suitable for transmitting the usable wavelengths and isunlikely to be significantly damaged by the particles which are directedback toward it at the distal end due to the aforementionedphotohydraulic phenomenon. If, however, damage should occur, the tip 30can be easily removed and replaced, as is commonly undertaken by adentist for various drill bits.

The optics combination of a meniscus lens 22 and an angular mirror 21,as shown in FIG. 10, can also be used with dynamic gas protection, asshown in FIG. 9. This combination of embodiments is shown in FIG. 11.Moreover, using a sapphire applicator tip 30, it is also possible toemploy the light waveguide material 17 protected by the metal jacket 24,extending directly to the direction-changing element, instead of thereflecting and focusing optics. Such an embodiment employing a prism 13as the direction-changing element is shown in FIG. 12, however, it willbe understood that an angular mirror, such as the mirror 21 can be usedin the embodiment of FIG. 12 instead of the prism 13.

In the removal of hard biological material, such as bone or dentalmaterial, the operating zone of the laser radiation should have aerosolapplied thereto intermittently, in accordance with the principles of theinvention. The aerosol drops are carried away as part of the removalprocess, but a portion of the aerosol drops will be vaporized by theheat, and thus function as heat extractors, so that it is possible tooperate at laser pulse repetition rates as high as 15 hertz without thethermally-damaged peripheral zone being any larger than that, forexample, arising with the use of an air-powered drill. It is surprisingto find that no loss in efficiency occurs, provided a given ratiobetween air and liquid is maintained, and the liquid volume is adjustedin accordance with the pulse energy and rate of repetition of the laserpulses. A significantly increased level of ablation quality is alsoachieved.

These measures make possible, for the first time, the ablation of hardtissue with a high degree of efficiency from an exact location point,while maintaining close temperature tolerances (less than 5° K.temperature increase). The apparatus connections for achieving the abovemethod results are shown in FIGS. 13 through 15.

The adaptation of the air-to-liquid ratio of the aerosol in accordancewith the invention is shown in FIG. 13, which also shows means foradjusting the ratio of the liquid proportion to the laser pulse energyand the rate of repetition of the laser pulses. For this purpose, areference data field 35 is created and stored as a matrix in a PROM orEPROM in an electronic control device 34. Pulse energy data are suppliedfrom a pulse energy memory to the control device 34, and data relatingto liquid volume and repetition rate are supplied to the control device34 from a liquid volume/repetition rate memory 37. The appropriateoperating point on the three-dimensional matrix formed in the data field35 is selected, and is used to control a valve 38 to regulate the liquidvolume.

In order to keep the applicator tip 14 as small and easy to handle aspossible, in a preferred embodiment the valve 38 is disposed prior tothe liquid feed to the applicator tip 14. To avoid the possibility ofafter flow of the liquid caused by placing the valve 38 so far upstreamfrom the termination of the liquid conduit 16 and by its possibleelasticity, phase-displaced low pressure pulses can be applied to theliquid in the liquid conduit 16. As shown in FIG. 14, this can beaccomplished by a second valve 39 operated inversely to 38, by a controldevice 40 preceded by an inverter. Even if the valve 38 is placed closerto the termination of the liquid conduit 16, or if a non-flexibleconduit is used, the valve 39 can be used as a pressure relief valve,and the external application of low pressure is then not necessary.

The timing diagram for controlling the aerosol is shown in FIG. 15. InFIG. 15, the various operating conditions (ON, OFF) for the laser pulsesare shown, as well as those for the valve 38 and for the low pressurevalve 39. As can be seen in FIG. 15, the liquid valve 38 is controlledphase displaced by a time t_(d) compared to the leading edge of thelaser pulses j. This is for the purpose of keeping the energy losseswithin the aerosol during the laser pulses as small as possible. Theliquid volume is controlled using the pulse-to-pause relationship(t_(wmin) to t_(wmax)). The low pressure valve 39 is switched on by apulse from the control unit 40 for a predetermined time t_(u), whichsuffices to reduce the speed of the flow of the liquid in the liquidconduit 16. The laser pulses are shown as having a duration t_(p), andhaving an exemplary pulse pause of 100 ms.

In the preferred embodiment using a pulsed Er:Cr:YSGG laser, the liquidis pure water. Any other liquid may be used, however, in accordance withthe principles of the present invention, which is suitable for theparticular application, and which has sufficient optical absorptionproperties at the working wavelength of the laser. At the same time, theuse of an optical hollow guide with gas cleansing provides for theprotection of the sensitive optical components. Moreover, the advantagesof handling ease and sterilization result. In the embodiment shown inFIG. 17, the mirror 1 is a completely reflective mirror and the beamagain passes through a semi-reflective mirror 5, and is focused by alens 41 onto the intensity smoothing medium, which in the embodiment ofFIG. 17 is an absorber film 4a. The energy contained in the spikes ofthe pulsed laser radiation drills a hole in the absorber film 4a, whicheliminates these spikes, so that a beam having a smooth intensityprofile with respect to time is coupled into the waveguide. The absorberfilm 4a may be of solid or fluid material, such as so-called"self-healing" material. For example, the absorber may be a liquid whichregenerates itself, i.e., closes the hole caused by a laser pulse, afterthe laser pulse is shut off. The absorber film 4a is a medium having anabsorption coefficient greater than 100/mm for the wavelength which isemployed in the laser system being used, the absorption coefficientbeing defined as 1/mm of distance. According to this definition,approximately 1/3 of the intensity (more precisely, 1/e=1/2.71) is stillpresent after 1 mm of distance, given an absorption coefficient ofunity. Given an absorption coefficient of two, this means that 1/3 ofthe intensity is still present after a distance of 1/2 mm.

All fluids which contain molecules with OH groups and which absorb laserlight at the specified wavelengths of the laser system may be used asfluid absorption materials for the film 4a. Distilled water, ethanol orglycerol may, for example, be used, given a wavelength of 2.94 μm.

Materials such as methacrylate-hydroxy ethylmethacrylate copolymer, poly(2,3-dihydroxypropylmethacrylate) and mica are examples of solidmaterials which can be used as the absorber film 4a. Whereas mica onlyabsorbs at a wavelength of 2.7 μm, the first two of the recited solidmaterials can be used for a wavelength of 2.78 μm as well as for awavelength of 2.94 μm. (It should be noted for the second-recitedcompound that the numerical designation "2,3" means that hydroxyl groupsare present at the second and third locations of the compound.)

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

We claim as our invention:
 1. An apparatus for ablating hard biologicalmaterials, comprising:a pulsed laser which emits pulsed laser radiationin the form of laser pulses each having a time-varying intensity profileand a fundamental frequency having an associated wavelength in awavelength range selected from the group of wavelength ranges consistingof 200 through 400 nm, 1.3 through 3 microns, and 9.0 through 11microns, said intensity profile containing intensity spikes which causegeneration of harmonics of said fundamental frequency in said pulsedlaser radiation; an optical waveguide which transmits said pulsed laserradiation along a propagation path to a treatment site; and means forcoupling said pulsed laser radiation to said optical waveguide includingan absorber film which smooths the intensity profile of said laserpulses with respect to time by removing harmonics in said pulsed laserradiation which exceed a predetermined threshold, and which permitspassage of pulsed laser radiation at said fundamental frequencytherethrough unchanged.
 2. An apparatus as claimed in claim 1 whereinsaid laser is a solid-state laser.
 3. An apparatus as claimed in claim 1wherein said laser is an Cr:Er:YSGG laser operating at a wavelength of2.78 microns.
 4. An apparatus as claimed in claim 1 wherein said laseris an Er:YAG laser operating at a wavelength of 2.94 microns.
 5. Anapparatus as claimed in claim 1 wherein said optical waveguide is formedby at least one optical fiber.
 6. An apparatus as claimed in claim 5wherein said at least one optical fiber consists of zirconium fluoride.7. An apparatus as claimed in claim 5 wherein said optical waveguidefurther includes an elastomeric inner jacket surrounding said at leastone optical fiber and a torsion resistant outer protective jacketsurrounding said inner jacket.
 8. An apparatus as claimed in claim 1further comprising a manipulable applicator for directing said pulsedlaser radiation onto a treatment site, wherein said optical waveguide isa flexible optical waveguide having a laser-proximate end and anapplicator-proximate end and has two rotary quick connectorsrespectively disposed at said ends for mechanically and opticallycoupling said laser-proximate end of said flexible optical waveguide tosaid laser and for optically and mechanically coupling saidapplicator-proximate end of said flexible optical waveguide to saidapplicator, each of said rotary quick connectors forming means forpreventing torsion forces to be transmitted to said flexible opticalwaveguide.
 9. An apparatus as claimed in claim 5 further comprising amanipulable handpiece optically coupled to said optical waveguide, saidhandpiece having an applicator endpiece from which said pulsed laserradiation exits onto a treatment site at which said pulsed radiationproduced ablation products, said applicator endpiece having surfaces,and said apparatus further comprising means for delivering media intothe propagation path of said pulsed laser radiation exiting from saidapplicator endpiece for preventing said ablation products produced fromthe treatment site by said pulsed laser radiation from damaging saidradiation transmitting surfaces of said applicator endpiece.
 10. Anapparatus as claimed in claim 9 wherein said applicator endpiececomprises:an optical hollow guide having an interior channel throughwhich said pulsed laser radiation proceeds to a treatment site; and agas container, connectable to a source of gas, and in fluidcommunication with said channel in said optical hollow guide forcreating a gas flow through said channel for preventing ablated materialfrom said treatment site from coming into contact with said applicatorendpiece.
 11. An apparatus as claimed in claim 10 wherein said gascontainer has a first side through which said pulsed laser radiationenters said gas container and a second side through which said pulsedlaser radiation exits said gas container, and said gas container having,at least on said first surface, a window closing said gas containerconsisting of material transmissive for said pulsed laser radiation. 12.An apparatus as claimed in claim 11 wherein said window consists ofmica.
 13. An apparatus as claimed in claim 9 wherein said applicatorendpiece consists of a solid conical sapphire tip.
 14. An apparatus asclaimed in claim 9 wherein said means for delivering media is a meansfor delivering an aerosol consisting of said media.
 15. An apparatus asclaimed in claim 14 wherein said means for delivering aerosol includesseparate means for delivering air and means for delivering fluid andmeans for mixing said air and fluid to form said aerosol, and means forsetting an air-to-fluid ratio and a fluid volume in said aerosol basedon the energy and repetition rate of the pulses of said pulsed laserradiation.
 16. An apparatus as claimed in claim 1 wherein said absorberfilm consists of self-healing material.
 17. An apparatus as claimed inclaim 1 wherein said absorber film comprises a solid material.
 18. Anapparatus as claimed in claim 17 wherein said solid material is selectedfrom the group consisting of methacrylate-hydroxy ethylmethacrylatecopolymer, poly (2,3-dihydroxypropylmethacrylate), and mica.
 19. Anapparatus as claimed in claim 1 wherein said absorber film consists of aliquid.
 20. An apparatus as claimed in claim 19 wherein said liquidcontains molecules with OH groups.
 21. An apparatus as claimed in claim19 wherein said liquid is selected from the group consisting ofdistilled water, ethanol and glycerol.